P
S urge arresters are commonly used in switchgear today to clamp transient voltages at levels below the allowable BIL (basic impulse insulation level) of the equipment being fed and the switchgear feeding it. The arrester plays a special part for motors and generators in extending the equipment life under certain system configurations. Metal oxide varistors (MOV) are commonly used in switchgear and medium voltage motor control centers (MV MCC). The metal oxide arresters offer smooth turn-on and turn-off at lower surge current levels than the older valve type silicon-carbide technology. To install MOV surge arresters in switchgear and MV MCC, there are several physical aspects to be considered to assure reliable operation. The most significant of these are ambient temperature, the lead length of the conductor connecting the arrester to the equipment being protected, and the spacing between the arresters in two separate phases.
The ambient temperature of the enclosure should be considered when determining the location of surge arresters. Metal oxide varistors are rated for a -40ºC to 40ºC ambient with a temporary maximum air temperature of 60ºC. As the temperature of the device increases the leakage current continues to climb. The increase in leakage current causes the surge arrester temperature to exceed the temperature rating of the MOV. The internal resistance of the surge arrester begins to drop off and the device turn-on voltage is lowered. If the temperature exceeds the thermal capability of the arrester, thermal runaway occurs and the arrester turns on at the system operating voltage and will not turn off, thus resulting in a line-to-ground fault.
It is critical that the length of the leads connecting the surge arrester to the protected equipment and the arrester to the ground bus be minimized. The wave continues to travel down the system past the surge arrester near the speed of light at the original surge voltage until the arrester turns on to clamp the voltage at the arrester discharge voltage. The voltage drop of the lead wire connecting the arrester to the equipment by Jim Bowen
Powell Electrical
Manufacturing Co.
being protected and the arrester to the ground plane adversely affects the discharge voltage of the surge arrester. A rule of thumb is that the arrester discharge voltage is decreased by 1.65 kV/ft. for every foot of lead length between the phase conductor and the ground bus. The decreased discharge voltage becomes more important in systems above 15 kV where BIL coordination margin is reduced below the 20 percent protection margin recommended by ANSI.
The surge arrester is dielectrically modeled as two dielectic materials in parallel for BIL purposes. This surface distance is commonly referred to as the creepage distance. The second part is the inner portion of the arrester, the
MOV disk, which is an insulator at nominal voltage and a conductor
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at higher voltages. To prevent conduction across the surface (tracking), adequate phase-to-phase and phase-to-ground spacing must be maintained. A skirt to skirt spacing of one inch for every 30 kV of BIL is recommended.
In addition to skirt to skirt spacing, it is important that phase spacing be maintained between the conductors throughout the lineup. The conductor connected to the lug on top of the arrester is at line potential and must maintain the same phase spacing as the bus phase conductors. (See the table below.). If the switchgear spacing is based on insulated phase distances, the lug connection on top of the arrester must be booted.
The boot fit should barely cover the first skirt ring and should form a continuous path from the conductor to the first ring of the arrester.
To keep from “shorting out” the creepage distance, the boot may not make contact with more than the first skirt.
These physical considerations play an important part in assuring that the arresters are able to function properly.
Rated
Maximum
Voltages
Low-
Frequency
Withstand
Impulse
Withstand
Air Clearance
Insulated
Conductors
Bare
Conductors
Surface Clearance
Insulated
Conductors
Bare
Conductors
635 V
4.76 kV
15 kV
27”
38 kV
2.2 kV
19 kV
36 kV
60 kV
80 kV
N/A
60 kV
95 kV
125 kV
150 kV
N/A
2”
3”
6”
7 1⁄2 “
1”
3 1/2”
6”
9”
10 1/2”
N/A
3”
5”
9”
11”
2”
5”
7”
14”
17”
Jim Bowen graduated from Texas A&M University in 1976 with a BSEE. He has worked for SIP Engineering as a power engineer and for Exxon in all facets of electrical engineering in the petrochemical process. He held the position of regional engineer for
Exxon Chemicals Europe for three years. In January of 1997, Jim joined Powell Electrical Manufacturing Company as Technical Director, providing leadership, training, and mentoring to both internal and external electrical communities.
2 NETA WORLD Spring 2006